Thermionic valve circuit arrangement



INVENTOR DOUGLAS FRANK GEORGE Kg v ATTORNEY Sept. 6, 1938. D. F. GEORGETHERMIONIC VALVE CIRCUIT ARRANGEMENT Filed May 2a, 1956 bib-h inhbhbbbb) 1111111 1111111'! Patented Sept. 6, 1938 h UNlTED f -ST'ATESPATENT orrlcc Douglas Frank George, Hutton, England, assignor to RadioCorporation of America, a corporation of Delawarev Application May 23,1936, Serial No. 81,361 In Great Britain May 24, 1935 4 Claims. (Cl.25027) This invention relates to thermionic valve 'cirotherwise knownunder the registered trade mark cuit arrangements and has for its objectto pro- Thyratron. Such a tube is normally prevented vide an improvedsafety circuit arrangement from being conductive by bias potentialapplied whereby the chances of damage to'a valve due to to its grid andderived from the grid circuit of the '5 flow therethrough of excessivecurrent caused by valve to be protected, the arrangement being such afailure in an associated circuit is substantially that when the gridcurrent of the valve to be pro reduced or eliminated. tected dropssubstantially the grid potential of The invention though not exclusivelylimited the Thyratron device changes to an extent, and thereto isparticularly applicable to radio transin a direction, sufficient tocause the said device to mitter and like high frequency oscillator valvecirbe'come'conductive, and thus bring down the po- 10 cuit arrangements.tential of the screen grid of the valve to be pro- If, in a radiotransmitter or like apparatus, a tected. thermionic valve be connectedin a high frequency The invention is illustrated in theaccompanycircuit-either as a high frequency oscillator or ing drawingwhich shows diagrammatically one as a high frequency amplifier driven bya preembodiment thereof. 15 ceding oscillator'of usual conventionarrange- Referring to the drawing, a high frequency ment, substantialgrid current will fiow under noramplifierfor example in a radiotransmitter 1 mal oscillator conditions. If, in the case of a comprisesa screen grid valve l the control grid high frequency amplifier valvethe oscillatory 2 of which is connected to the cathode point 3 drivefails, orin the case of ahigh frequency osthrough a grid resistance 4.The screen grid 5 20 cillator valve some high frequency component in ofthe valve is connected to the anode ll of a gas the circuit breaks down,the valve will no'longer filled grid controlled discharge device l whosepresent its normal back electro-motive force, and cathode point 8 isconnected to the cathode point in consequence, will take. an abnormallyheavy 3 of the screen grid valve I. A suitably chosen anode currentwhich will in time damage the tapping 9 upon the grid resistance t isconnected 25 valve, e. g. will cause it to become sof or lose to thegrid ill of the grid controlled discharge deits cathode emission. As afurther consequence, vice 1. The anode ll of the valve H is conthenormally substantial grid current will cease nected through a couplingcondenser l2 to one end to flow. The present invention utilizes thislatter of the usual parallel tuned output circuit l3 the effect toprovide an automatic'safety action preother end of this circuit beingconnected to the 30 venting the continued flow of abnormally high commoncathode point. High frequency oscillaanode current due to a breakdown.tions from a suitable master or other drive In carrying out thisinvention, the valve to source (not shown) are applied, throughasuitable be protected is provided with a screen grid (i. e. couplingcondenser M to the control grid 2 of the it is a valve having a gridbetween the control valve l and anode potential from a source (not 35grid and the anode, e. g. a so-called screen grid shown) is applied tothe anode H of the valve l valve, pentode or other multi-grid valve) andthrough a choke l5. Normal relatively high means Operated automaticallyin dependence p screen grid potential is applied to the screen grid thegrid current of the said valve is provided for 5 of the valve I from theanode potential source by 40 substantially lowering the screen gridpotential means of a high resistance potentiometer con- 40 of the valvebelow normal if the said valve ceases sisting of two series resistancesit, it, one It) to pass normal grid current. It has been found of whichis connected between the potential source that if the screen gridpotential of a valve havand the screen grid 5 and the other ll) of whiching a screen grid be considerably reduced in is connected between thescreen grid 5 and the value from its normal high positive potential, thecommon cathode point. 45 internal anode-cathode resistance of the valvein- The whole arrangement is such that in normal creases to a high valuewhich, at zero screen grid operation the voltage applied to the grid Ill of the potential, approaches infinity. Accordingly the gridcontrolled gas filled discharge device 71 is effect of automaticallyreducing the screen grid sufficient to prevent said device from becomingpotential is substantially to reduce the anode curconductive, but if-for7 example owing to the 5 rent which can flow through the valve. faiureof the high frequency driving source Preferably the automatic reductionOf th the grid current of the screen grid valve drops Screen ridpotential s Obt d y Connecting in much below normal, the negative biasvoltage on the screen grid circuit a gas filled electric disthe grid ofthe gas filled discharge device is no charge device of thetrigger-action type, or a tube longer maintained at a sufficient valueto prevent 55 said device from becoming conductive, and accordingly thesaid device becomes conductive thereby bringing down the voltage of thescreen grid 5 to much below normal value. Accordingly the internalresistance of the valve l rises to an extent amply sufficient to preventit from damage due to excessive anode current flow.

By suitably choosing the gas filled discharge devicesuch devices givingan anode to filament voltage drop of only about 15 volts when current isbeing passed, are available on the market at the present timeit ispossible to cause the screen grid potential to be reduced almost to thatof the cathode, thus reducing the anode feed current of the screen gridvalve practically to zero. By virtue of the fact that the resistancesI6, I! are high, the high tension current flow through the gas filleddischarge device, when the said device is conductive, will be vary low.

The invention is not limited to the use of a gas filled discharge deviceor other purely electrical relay, since obviously a suitableelectromechanical relay could be employed and operated under gridcurrent control, but the illustrated embodiment with its purelyelectrical action is preferred. Again, as will be obvious, the invention can be employed to protect a bank of valves just as well as asingle valve.

An important practical advantage of the present invention lies in itsextreme simplicity and cheapness of construction.

The following statement of components (this statement is given purely byway of example) relates to a circuit as illustrated and which has beensuccessfully employed in actual experimental practice.

Valve l: Valve as now commercially known under the trade designationMarconi A. C./S. 2.

Anode voltage: 3,500 volts.

Resistance l6: 45,000 ohms.

Resistance I1: 12,000 ohms.

Resistance 4: 12,000 ohms between grid 2 and point 9. 7 1,800 ohmsbetween point 9 an point 3.

Device 1: Gas filled relay as now commercially known under the tradedesignation G. T. 1.

In this embodiment, under normal oscillatory conditions the anode feedwas 250 milli-amperes, and the total anode power was 875 watts with thegrid 2 at 200 volts negative, 750 volts (maximum) on grid 5, 26 voltsnegative on grid l0,

and 750 volts on anode 6, there being, of course, no discharge currentthrough the device I.

On the occurrence of a failure (e. g. a failure of carrier frequencydrive to the valve I) the anode feed, the total anode power, thevoltages on the grids 2, 5 and I0, all dropped practically to zero andthe voltage on anode 6 became approximately 15 volts. Without the safetycircuit, a similar fault would have destroyed the valve l by anode deadloss wattage.

What is claimed is:

1. A protective system for a circuit of the type which includes anelectron discharge tube for the generation of oscillations, said tubehaving a cathode, an anode, a control grid and at least one other grid,said protective system comprising a gaseous discharge tube of thetrigger-action type, a control grid in the last said tube, meansinter-connecting the controlgrids of the two said tubes for causing thegaseous discharge tube to be triggered off in response to a cessation ofoscillations in the first said tube, and means acting in dependence uponthe conductivity of the gaseous discharge tube for rendering the firstsaid tube substantially non-conductive.

2. A system in accordance with claim 1 and further characterized in thatthe first said discharge tube possesses a screen grid electrode and saidgaseous discharge tube possesses an anode electrode, these twoelectrodes being inter-connected.

3. A protective system for a multi-grid electron discharge tube on theinput circuit of which oscillations are normally impressed, said systemcomprising a grid-controlled gaseous discharge tube having a space pathof relatively low impedance when triggered off, means including aresistive impedance in said input circuit of the first said tube and aconnection therefrom to the control grid of said gaseous tube fortriggering ofi the latter in response to a marked diminution ofamplitude of said oscillations in said input circuit, and means actingin dependence upon a flow of space current in said gaseous tube forrendering the first said tube substantially nonconductive.

4. A system in accordance with claim 3 and having an output circuit forsaid gaseous tube which connects with a screen grid in said multigriddischarge tube.

DOUGLAS FRANK GEORGE.

